I wouldn't be against the WWE letting Orton retain at Mania. In fact, I think it would be a great move at this point in time.
Orton is a star and a main event level guy, but he hasn't quite broken into the ranks of a top level draw type guy yet. A win, in a Mania title match would do wonders for him.
I know they rarely let heels win title matches at Mania because they like to send the crowd home happy, but they can still do that if they put Taker vs Edge last.
One thing they could also try doing is a double turn, which we haven't seen in a while. Orton still gets the occasional cheer and I still beleive he can become a great face if things are pulled off right. Especially after the face like booking he was given last Monday. If they can build on this, where Orton becomes less of a chicken shit and starts dominating cleanly it can work. The thing is, I doubt they will turn Cena and it would be hard to turn HHH, outside of using Vince McMahon which they have done before at Mania.